1. Field of the Invention
The invention relates to the field of combustion technology. It relates to a device for operating an annular combustion chamber for gas turbines which is equipped with combined burners for liquid and gaseous fuels, in particular burners of the double-cone design, each burner having an airblast nozzle for atomizing the liquid fuel, into which airblast nozzle atomizing air is fed from outside the burner hood, and the inflow of the atomizing air being at least partly throttled during gas operation.
2. Discussion of Background
To achieve the lowest possible NOx emissions, burners are operated close to their lean extinction limit. As a result, however, the control range of the burners is greatly restricted. In order to remove this disadvantage, internal piloting is used, for example, during part-load operation of a gas turbine, during which piloting the fuel gases are enriched with additional fuel near the axis. The stability range of the burners is thereby extended so far that reliable operation is guaranteed even under part-load conditions.
To operate a burner alternatively with gaseous or liquid fuel, it is known to atomize the liquid fuel by means of an airblast nozzle. To this end, air is injected near the axis, i.e. in the center of the burner. In the same way as the combustion air in premix burners of the double-cone design, the basic construction of which is described in U.S. Pat. No. 4,932,861 to Keller et al., this atomizing air is extracted from a plenum outside the burner.
However, the injection of the atomizing air takes place not only during the fuel-oil atomization but also in gas operation, during which, however, no air at all is required for the atomization. In gas operation of the burner, this additional air destabilizes the flame, on the one hand due to the mixture being made leaner and on the other hand due to the incident flow itself. This leads in turn to a distinct reduction in the lean extinction limit of the gas flame.
DE 44 24 599 A1 discloses a method and a plurality of devices for operating a combined burner for liquid and gaseous fuels, in which this disadvantage is removed by provision being made to control the inflow of the atomizing air, the inflow of the atomizing air being at least partly throttled during operation with gaseous fuel.
To this end, according to DE 44 24 599 A1 the airblast nozzle, for example, is designed in such a way that the pilot-gas passage leads upstream of the atomizing cross section into at least one atomizing-air passage so that the throttling of the atomizing air is effected by means of the pilot gas. This embodiment has the disadvantage that, during operation of the burner with gaseous fuel, work has to be carried out with additional pilot gas not only during the starting phase but constantly.
In another embodiment described in DE 44 24 599 A1, in which the burner is fastened in the burner hood by a burner support having an integrated air-inlet opening for the blast air, and a fuel lance adjoins the burner support upstream for the feed of the liquid fuel, an adjusting mechanism, e.g. in the form of a displaceable or rotatable sleeve, is arranged on the fuel lance or the burner support, which adjusting mechanism at least partly closes the air-inlet opening for the atomizing air during operation of the burner with gaseous fuel. This solution has the disadvantage that the adjusting mechanism is arranged in the interior of the machine and is therefore difficult of access.
GB 2 091 409 A discloses an airblast nozzle in which the inflow of the combustion and atomizing air is jointly controlled in accordance with the demand of the combustion chamber. To this end, two different metering valves are arranged which are acted upon by a common piston. This solution has the disadvantage that no separate control of the atomizing air is possible. In addition, the valves are arranged directly on the nozzle and are therefore exposed to high temperatures.